Oh, yes, a cat can slip a harness quite quickly. If your kitty gets panicky the best thing to do is to keep the leash slack. I actually use a long rope that is about 15 feet long and keep half of it rolled up in my hand just in case I need to offer some slack.

Successful walking of your cat depends entirely on your cat. I could walk Snowball like a dog, except at a much slower pace, even take him to provincial parks to walk the trails. We would walk him around the subdivision without a leash (I don't recommend this for most cats, he was special) and he would just follow us. Some cats can't be trained to walk on a harness.